Radiographic images are produced by the detection of radiation that is transmitted through or scattered from the object being inspected. The density, atomic number and the total amount of material that is present determine the extent of attenuation of the radiation and, therefore, the nature and type of radiographic image produced. By determining the average absorption of the X-ray or gamma ray photons as they travel along the various X-ray paths, it is possible to derive information about the characteristics of the material through which they pass. The intensity of scattered X-rays is related to the atomic number (Z) of the material scattering the X-rays. In general, for atomic numbers less than 25, the intensity of X-ray backscatter, or X-ray reflectance, decreases with increasing atomic number. On the other hand, materials with high atomic number (Z>70) are characterized by high attenuation of the low and high end of the X-ray spectrum. Therefore, the X-ray images are primarily modulated by variation in the atomic number various materials present inside an object (such as within cargo).
As the final image is modulated in accordance with the atomic numbers of various materials present inside an object, it is common for X-ray imaging systems to produce images with dark areas. Although these dark areas might indicate the presence of threat materials, they yield little information about the exact nature of threat. In addition, the radiographs produced by conventional X-ray systems are often difficult to interpret because in these radiographs, the objects are superimposed which may confound the image. Therefore, a trained operator must study and interpret each image to render an opinion on whether or not a target of interest, such as a threat, is present. Operator fatigue and distraction can compromise detection performance when a large number of such radiographs are to be interpreted, such as at high traffic transit points and ports. Even with automated systems, it becomes difficult to comply with the implied requirement to keep the number of false alarms low, when the system is operated at high throughputs.
One method of obtaining more useful information and clarity from X-ray imaging is using dual energy systems to measure the effective atomic numbers of materials in containers or luggage. Here, the X-ray beam is separated into two broad categories: low energy X-ray beam and high energy X-ray beam. Often this is achieved by passing the X-ray beam through a first thin X-ray detector that responds preferentially to low-energy X-radiation. This filtered beam is then passed to a second detector, which responds to the remaining X-ray beam, which is weighted towards the higher energy part of the spectrum. Effective atomic number is then determined by taking the difference between the high energy and low energy signals. This method is particularly effective for X-ray energy beams in the range of 60 kV to 450 kV where the rapid change in linear attenuation coefficient of the object under inspection gives good contrast between the low and high-energy spectral regions.
Some of the challenges in processing high and low energy signals in a dual energy system, which, in turn, affect the accuracy of the calculated result, include varying angles at which the transmitted X-rays impinge upon the detectors and also the varying order in which the transmitted X-rays pass through the high and low energy detectors.
Accordingly, there is a need for improved method and system for signal processing in dual energy imaging systems, that addresses the challenges faced by conventional methods of signal processing and provides not only for high resolution in the generated images but also for better penetration performance.